Jason Day withdraws from championship at critical time

Jason Day has pulled out of the BMW Championship at one of the most critical times in the season. Photo / AP

World No.1 Jason Day's place in the lucrative season-ending Tour Championship is in doubt after he withdrew from the BMW Championship with a back injury through just eight holes of the final round.

Day tweaked his lower back when he bent down to put a tee in the ground on the ninth, deciding to pull out of the event rather than risk further damage.

The 10-time PGA tour winner and defending champion at the event has a history of back issues, withdrawing from the BMW Championship in 2014 with a similar complaint.

He also battled a bad back at the 2015 Barclays and 2016 World Golf Championships Match Play before fighting through the pain and winning.

His place in Atlanta in two week's time, where the winner of the season long FedExCup will claim the $US10million bonus, is now in doubt.

"He's all right, he's just got some back issues and is in a little bit of pain. He's getting treatment right now. I don't think it's anything that he hasn't dealt with before," said Day's manager Bud Martin.

"The trainer is maybe thinking it's the same type of thing that he dealt with at the Barclays and last year where he picked something up in his bus and it kind of got out of line.

"But just better safe than sorry. Instead of forcing it, we just decided that it was more prudent to come in, get it worked on, and deal with it."

Martin would not speculate whether Day would be at full strength in time for Atlanta. Day started the week at 2nd on the FedExCup points list, hopeful of remaining inside the all-important top 5 headed to East Lake.

Those in the top 5 can guarantee the FedExCup with a win in the Tour Championship. With zero points he is now projected fourth but could potentially fall as low as 6th.

"We'll probably get an MRI just to double check. My gut is that he'll be fine," Martin said. "Hopefully, he'll be ready for Atlanta, but the most important thing is the next 15 years, not the next 15 days."